LeetCode-in-Java

1483. Kth Ancestor of a Tree Node

Hard

You are given a tree with n nodes numbered from 0 to n - 1 in the form of a parent array parent where parent[i] is the parent of ith node. The root of the tree is node 0. Find the kth ancestor of a given node.

The kth ancestor of a tree node is the kth node in the path from that node to the root node.

Implement the TreeAncestor class:

• TreeAncestor(int n, int[] parent) Initializes the object with the number of nodes in the tree and the parent array.
• int getKthAncestor(int node, int k) return the kth ancestor of the given node node. If there is no such ancestor, return -1.

Example 1:

Input [“TreeAncestor”, “getKthAncestor”, “getKthAncestor”, “getKthAncestor”] [[7, [-1, 0, 0, 1, 1, 2, 2]], [3, 1], [5, 2], [6, 3]]

Output: [null, 1, 0, -1]

Explanation:

TreeAncestor treeAncestor = new TreeAncestor(7, [-1, 0, 0, 1, 1, 2, 2]);

treeAncestor.getKthAncestor(3, 1); // returns 1 which is the parent of 3

treeAncestor.getKthAncestor(5, 2); // returns 0 which is the grandparent of 5

treeAncestor.getKthAncestor(6, 3); // returns -1 because there is no such ancestor

Constraints:

• 1 <= k <= n <= 5 * 104
• parent.length == n
• parent[0] == -1
• 0 <= parent[i] < n for all 0 < i < n
• 0 <= node < n
• There will be at most 5 * 104 queries.

Solution

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class TreeAncestor {
    private final List<Integer> steps;
    private final Map<Integer, int[]> stepMap;

    public TreeAncestor(int n, int[] parent) {
        steps = new ArrayList<>();
        stepMap = new HashMap<>();
        steps.add(1);
        stepMap.put(1, parent);
        int stepBase = 10;
        int step = stepBase;
        while (step * 2 < n) {
            int[] stepArr = new int[n];
            int[] lastStepArr = stepMap.get(steps.get(steps.size() - 1));
            for (int i = 0; i < n; i++) {
                int cur = i;
                for (int repeat = 0; repeat < stepBase && cur != -1; repeat++) {
                    cur = lastStepArr[cur];
                }
                stepArr[i] = cur;
            }
            steps.add(step);
            stepMap.put(step, stepArr);
            step *= stepBase;
        }
    }

    public int getKthAncestor(int node, int k) {
        int index = steps.size() - 1;
        while (k > 0 && node != -1 && index >= 0) {
            int step = steps.get(index);
            int[] stepArr = stepMap.get(step);
            while (k >= step && node != -1) {
                node = stepArr[node];
                k -= step;
            }
            index--;
        }
        return node;
    }
}

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